1. Field of the Invention
The present invention relates generally to a dynamic refresh operation for optical data storage, and more particularly pertains to a method for dynamically refreshing a holographic data storage medium. The data storage medium can comprise crystals of CdF.sub.2 doped with Ga which can record holograms by the "DX effect", in which optical excitation causes a long term change in the refractive index, on the order of 0.01, to achieve very high volume storage bit densities. The present invention can advantageously be utilized in memory applications where hard disc drives are currently utilized, such as to load software, wherein relatively little writing and erasing operations occur.
2. Discussion of the Prior Art
The present invention relates to the field of holographic data storage. A holographic data storage arrangement enables a very high storage capacity and very high recording and readout rates compared to existing disk-based techniques, and can be used within computer systems as an addition to, or replacement for, conventional magnetic disk or tape or magneto-optic storage systems.
The holographic recording material medium is typically formed of a material such as lithium niobate, or a DX-center material. Typically, the holographic medium in a prior art holographic storage system has the shape of a cube or cuboid, or is cylindrically shaped, and is fixed in position. The optical beam bearing the data image is typically incident from a fixed direction. A single hologram is recorded by allowing an image beam containing a two-dimensional page of data to be incident on the medium simultaneously with a simple write reference bead derived from the same source as the data image beam, but incident at a different angle. The two beams form a stationary optical interference pattern which is recorded by the holographic medium in the form of a refractive index image which follows the interference pattern.
Reconstruction of the data image is achieved by illuminating the holographic medium with a read reference beam identical to the write reference beam, causing Bragg diffraction to reproduce the original data image beam.
Illumination with a read reference beam identical to the write reference beam in every respect except its direction results in negligible reconstruction of the data page. The angular range of the read reference beam around that of the original write reference beam which may be used and still cause readout of only the single desired data page is a function of the physical parameters of the system. This angular proximity is referred to as the angular Bragg width. This angular selectivity allows recording of multiple holograms of data pages by changing the angle of incidence of the write reference beam. Other methods of multiplexing pages are possible, such as by the use of varying wavelengths, or by spatial encoding of non-plane reference beams, but angular multiplexing of a simple plane wave reference beam is the most common approach.
Typically, a laser emits a beam which is collimated by a collimating lens and split by a beamsplitter to form a reference beam and a data beam. The reference beam is redirected by a prism or mirror to fall onto the holographic medium. The data beam is redirected by a prism or mirror to fall upon a page-composing device such as a spatial light modulator (SLM) which imposes a two-dimensional pattern of the data page to be stored onto the amplitude of the beam. This pattern typically consists of a dark region (or "pixel") representing a binary zero bit and a bright pixel representing a one bit.
A lens forms either an image or a Fourier transform of the data page within or near the holographic medium, where it interfaces with the reference beam to form a holographic recording of the data page. The two beams are incident on the holographic medium for some exposure time which is such that the required magnitude of change of refractive index is created. This time depends upon the laser power available and the sensitivity of the holographic medium.
To read out data, the page-composing device is turned off to block all light in its path, and only the reference beam falls on the holographic medium. The data page beam is reproduced by diffraction from the stored hologram, and is imaged (or Fourier-transformed) by a lens to create an image of the original data page which may be recorded by a camera. The electrical signal retrieved from each camera pixel is discriminated to be either a zero or a one, and the original digital data is thus retrieved.
The inherent theoretical capacity of a holographic storage medium in bits is V/.lambda..sup.3 (according to a calculation by P. J. Van Heerden, Appl. Opt., 2(4), p. 393, (1963), where V is the volume of the holographic material and .lambda. is the wavelength of light inside the holographic material. This leads to very high potential memory capacities and is a major reason for the interest in holographic storage. For example, a 1 inch cube with a refractive index of 1.5 has a theoretical capacity of 4.4.times.10.sup.14 bits at 500 nm wavelength, or 55 terabytes. In order to approach this value in practice, page sizes should be as large as possible (in the range 10.sup.6 to 10.sup.8 bits), and the maximum number of pages should be stored which the medium can support.
Since pages can be written and read out in parallel, data rates are also inherently high, and are typically limited by the speed at which data can be loaded into the page-composing device (during writing) or downloaded from a camera (during readout). For example, a page-size of 4.times.10.sup.7 bits need only be read out once every second to be equal to existing magnetic hard disk readout rates of 5 MB/s.